A fluororesin film is excellent in weather resistance, stain resistance, etc. and is therefore used as a roof material, a wall covering material, etc. for buildings/architecture such as outdoor exhibition halls, sports facilities and agricultural greenhouses (a building/architecture wherein a fluororesin film is used for the roof material, the wall covering material or the like will sometimes be referred to as a membrane structure). However, a fluororesin film has a high visible light transmittance and accordingly when it is used as a roof material, a wall covering material or the like, the interior may be too bright or the inside temperature may increase too much, and accordingly it is required to suppress the visible light transmittance.
As a method of suppressing the visible light transmittance of a fluororesin film, a method of reflecting the visible light and a method of absorbing the visible light may be mentioned. Among them, by the method of absorbing the visible light, the temperature of the fluororesin film tends to increase, whereby mechanical strength will decrease. Accordingly, the method of reflecting the visible light is preferred.
As a method of reflecting the visible light entering the fluororesin film, a method of forming on a resin film a coating layer containing a white pigment such as a titanium oxide pigment having a high refractive index may be mentioned.
However, a fluororesin film having a coating layer containing a titanium oxide pigment formed on the resin film has the following drawbacks.
i) Titanium oxide is likely to undergo discoloration and decomposition by ultraviolet rays. As a result, the fluororesin film is likely to undergo discoloration or a change in the visible light transmittance.
(ii) The fluororesin film and the coating layer are likely to undergo discoloration or a decrease in mechanical strength by photocatalytic reaction of titanium oxide by ultraviolet rays. As a result, the fluororesin film is likely to undergo discoloration, a change in the visible light transmittance or a decrease in mechanical strength.
(iii) Hydrogen fluoride tends to be generated by decomposition of the fluororesin by photocatalytic reaction of titanium oxide, and another color pigment blended in the coating layer together with the white pigment undergoes discoloration by hydrogen fluoride. As a result, the fluororesin film is likely to undergo discoloration or a change in the visible light transmittance.
As a pigment which overcomes the above problem (i), the following pigment has been proposed.
(1) A pigment which is a pigment comprising cerium oxide and dense amorphous silica precipitated on the surface of rutile-type titanium oxide, having an amount of cerium oxide of from 0.01 to 1.0 mass % of titanium oxide, and an amount of the dense amorphous silica of from 1 to 8 mass % of titanium oxide (Patent Document 1).
(2) A pigment which is a pigment comprising rutile TiO2 particles having a covering comprising alumina or alumina/silica, and which has cerium cations in an amount of from 0.5 to 2 wt % of TiO2 and sulfate anions, phosphate anions or silicate anions in a stoichiometric amount bonded to the particle surface (Patent Document 2).
However, the present inventors consider as follows. That is, with respect to the above pigment (1) or (2), although discoloration and decomposition of the pigment itself by ultraviolet rays are suppressed, the photocatalytic reaction of the pigment is not sufficiently suppressed. Particularly, its use together with a fluororesin is not considered, countermeasures against hydrogen fluoride are not taken. Accordingly, the problems (ii) and (iii) are not overcome, and the fluororesin film will undergo discoloration, a change in the visible light transmittance and a decrease in mechanical strength.
On the other hand, heretofore, a titanium oxide pigment has been used as a white pigment for a coating material. Various proposals have been made with respect to improvement in weather resistance of a coating layer formed from a coating material containing a titanium oxide pigment. However, a white coating material capable of forming a coating layer excellent in weather resistance and acid rain resistance so that brightness of the coating layer will not be impaired particularly after exposure to the outdoors for 10 years or longer has not been obtained. For example, in a case of forming a coating layer on a metal substrate, adhesion between the coating layer and the metal substrate will be maintained even after long exposure to the outdoor. However, by photocatalytic reaction of titanium oxide by ultraviolet rays, discoloration or cracking on the surface layer of the coating layer may occur, or a resin binder and the pigment may be peeled off from the surface layer of the coating layer.    Patent Document 1: JP-A-7-315838    Patent Document 2: JP-A-59-184264